A phased-array antenna is a directive antenna having several individual, suitably-spaced radiating antennas, or elements. The phased array generates a radiation pattern ("beam") having a main lobe and side lobes that is determined by the collective action of all the radiating elements in the array. The response of each radiating element is a function of the specific phase and amplitude of a signal applied to the element. By varying the relative phases of the signals applied to the individual radiating elements, the beam can be advantageously changed in azimuth ("beam steering"), elevation ("beam tilting") or both.
Beam steering/tilting has a number of applications. Of major significance is its application to the field of wireless telecommunications. The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially-distinct areas called "cells." Each cell usually has an irregular shape (though idealized as a hexagon) that depends on terrain topography. Typically, each cell contains a base station, which includes, among other equipment, radios and antennas that the base station uses to communicate with the wireless terminals in that cell. Due to instantaneous geographic variations in communications traffic, it may be desirable, at times, to adjust the geographic coverage of a particular base station. This can be accomplished by beam steering/tilting.
There are a variety of different ways to obtain a relative phase change between the signals applied to the various antenna elements for beam steering/tilting. The change in phase .phi. experienced by an electromagnetic wave of frequency f propagating with a velocity v through a transmission line of length l is given by the expression: .phi.=2.pi.fl/v. As is well known to those skilled in the art, the velocity v of an electromagnetic wave is a function of the permeability .mu. and the dielectric constant .epsilon. of the medium in which the wave propagates. Thus, phase can be changed by altering frequency, line length, propagation velocity, permeability or dielectric constant.
Devices for causing a differential phase change ("phase shifters") utilizing the aforementioned phase-shifting techniques are known. One type of phase shifter utilizes switchable delay lines having different lengths. Such a phase shifter is usually big and expensive. Moreover, due to the discrete nature of such a device, an error in desired phase will typically be present. A second type of device is a solid-state hybrid-coupled-diode phase shifter. Such devices suffer from high insertion loss and nonlinearity. As a result of such high insertion loss, amplifiers are required at the top of a base station tower to increase signal levels. At the high power levels required for transmission, such amplifiers are heavy, big and expensive. Such amplifiers are considerably smaller and less expensive at "receive" power levels, although it is still generally undesirable to have such active RF electronics at the top of a tower.
A third type of phase shifter uses a ferrimagnetic material (a ferrite). It is known that the permeability of a ferrite can be changed by varying an applied D.C. magnetic field. Such a permeability change results in a change in the propagation speed of an electromagnetic wave traveling through the ferrite, resulting in phase shift. Traditionally, ferrite phase shifters have been quite large, heavy and expensive. More recently, thin-film ferrites has been utilized for such shifters, which reduces their size and weight. Such thin-film-based ferrite phase shifters disadvantageously become nonlinear, however, at high power levels. A fourth type of phase shifter utilizes a "sliding contact" technique. In one implementation of a sliding-contact phase shifter, coaxial lines "telescope" into or out of one another such that the line length of the phase shifter, and hence the phase imparted thereby, is changed. Such phase shifters, commonly referred to as "line-stretcher" phase shifters, suffer from corrosion and electrical contact problems over time.
Due to the explosive growth of wireless communications, there is a growing need for steerable/tiltable linear phased-array antennas. To meet that need, it would be desirable to have a phase shifter that avoids the drawbacks of the prior art.